CONDENSATION

You will need:
Glass of water 

Procedure:

1. Put the glass of water in the refrigerator for about an hour or so, until it is very cold. 
2. Take it out of the refrigerator after it is cold, and notice how drops of water form on the outside of the glass.

Explanatory Note
The glass filled with cold water cools the air around it when you take it out of the refrigerator, and some of the water vapor in the air condenses and forms drops of water on the outside of the glass